1. Field of the Invention
The invention relates to a display apparatus and its pixel structure, and more particularly, to a current-driven organic light emitting diode (OLED) display apparatus and its pixel structure.
2. Description of the Prior Art
Referring to FIG. 1, which is a diagram of a conventional pixel 10 of a voltage-driven OLED display apparatus. As shown in FIG. 1, the pixel 10 comprises a scan line SL, a data line DL, a thin-film transistor (TFT) M1, a thin-film transistor M2, a capacitor C, and an organic light emitting diode (OLED). The gate of the TFT M1 is connected to the scan line SL, the drain of TFT M1 is connected to the data line DL, and the source of the TFT M1 is connected to the gate of the TFT M2 and the capacitor C. The drain of the TFT M2 is connected to the organic light emitting diode (OLED), and the source of the TFT M2 is connected to the capacitor and a voltage source Vdd. Furthermore, the organic light emitting diode (OLED) is connected to another voltage source Vss.
In addition, the operation of the pixel 10 is illustrated as follows. First of all, an external gate driver (not shown) drives the scan line SL and supplies a predetermined voltage to the scan line, the predetermined voltage is transferred to the gate of the TFT M1 through the scan line SL, and the TFT M1 is utilized as a switch. Therefore, the TFT M1 is turned on. In addition, the voltage information carried by the data line DL can be transferred to the gate of the TFT M2 and the capacitor C through the TFT M1. Please note that the voltage information carried by the data line DL is set by the external data driver (not shown) according to the display data (for example, a gray value of the pixel 10) to be displayed of the pixel 10.
And then, because the above-mentioned voltage information is utilized to control the gate voltage of the TFT M2, the TFT M2 can determine the current I, which passes through the TFT M2, according to the voltage information. On the other hand, because the luminace of the organic light emitting diode (OLED) is directly proportional to the current I, the organic light emitting diode (OLED) generates a corresponding amount of light according to the current I, and the pixel 10 is driven.
As shown in FIG. 1, the capacitor C is utilized to store the above-mentioned voltage information. When the voltage information passes through the TFT M1, the voltage information is not only utilized as the gate voltage of the TFT M2 for turning on the TFT M2, but also affects the charges stored in the capacitor C. Therefore, when the capacitor C stores enough charges for maintaining the voltage level corresponding to the above-mentioned voltage information, the gate driver and the data driver can stop driving the pixel 10. And then the capacitor C can be utilized to continuously drive the TFT M2 to make the TFT M2 output the current I for a predetermined time interval. Furthermore, because the capacitor C is utilized to drive the TFT M2, noise from data line DL no longer affects the TFT M2. Therefore, this can make the organic light emitting diode (OLED) stably generate light. In other words, the pixel 10 can stably output a wanted gray value.
However, inaccuracies in manufacturing the TFT M2 (for example, an inaccurate doping concentration or an inaccurate distance between the gate and the substrate) may occur. This may cause an inaccuracy of the threshold voltage of the TFT M2 or an inaccuracy of the mobility of the TFT M2. These inaccuracies may directly affect the current I. Therefore, even if the same voltage information is utilized, currents I of different pixels are still different. In other words, this makes different pixels having the same voltage information display with different luminance values.